1. Field of the Invention
The present invention relates to a charge pump circuit for use in a power supply circuit, and more particularly to a charge pump circuit which can efficiently power a load that requires a voltage much higher than a supply voltage. The present invention also relates to a power supply circuit using a charge pump circuit which can improve reliability when it is used for a load which presents large fluctuations in current consumption, and ensure safety even in the event of short-circuiting to a ground voltage.
2. Description of the Related Art
When a voltage higher than a supply voltage is required, an inductance based DC-DC converter is mainly used as a power supply circuit. The DC-DC converter is used in many applications because it can generate an arbitrary voltage and efficiently power a load which consumes a large current. However, since the DC-DC converter requires a number of parts such as a transformer, a coil and the like, it is difficult to reduce the size of the DC-DC converter, and all the parts of the DC-DC converter cannot be integrated into a semiconductor integrated circuit.
For the reason set forth above, a highly efficient charge pump circuit which can be reduced in size is used for a power supply circuit for powering a load which consumes a relatively small current. However, since the charge pump circuit adds voltages on capacitors charged by a supply voltage from a DC power supply for boosting, the charge pump circuit can merely provide output voltages which are integer multiples of the supply voltage. Depending on the relationship between the supply voltage and a voltage required by a load, the load is supplied with an unnecessarily high voltage, resulting in larger power consumption of the load and a significantly lower efficiency.
Laid-open Japanese Patent Application (JP-A-) No. 2001-169537 discloses a charge pump circuit which improves a drawback of the charge pump circuit which merely provides output voltages that are integer multiples of a supply voltage. Specifically, two capacitors having the same capacitance are disposed closest to a power supply. The two capacitors are connected in series and charged by a supply voltage, so that each of the capacitors is charged to one half of the supply voltage.
The voltage generated by the two charged capacitors connected in parallel is added to a voltage on another capacitor which is charged at the same voltage as the supply voltage or to the supply voltage to generate a voltage (N+0.5) times as high as the supply voltage, where N is an integer larger than zero (N>0). JP-A-2001-169537 also discloses a circuit which comprises four capacitors disposed closest to a power supply, and can set output voltages in increments of a quarter of the supply voltage by adding voltages on the four capacitors connected in parallel.
In general, conventional charge pump circuits employ a plurality of diodes for preventing a current from flowing back from a high potential side of a capacitor charged at a high voltage to the DC power supply. Therefore, a loss caused by forward voltages of the diodes degrades the efficiency to a non-negligible extent, particularly when the supply voltage is low. Although the charge pump circuit disclosed in JP-A-2001-169537 can boost the voltage in small increments of one quarter of the supply voltage, this charge pump circuit also employs diodes for preventing a current from flowing back from a high potential side of the capacitors charged at a high voltage, seemingly causing a loss due to the forward voltages of the diodes.
In addition, a switching element for connecting the capacitors in series includes a MOS transistor which has a substrate gate connected to a source. Assuming that these circuits are integrated into a single IC chip, when the MOS transistor has a source voltage higher than a drain voltage in the middle of a boosting operation, a forward current flows into a parasitic diode of the MOS transistor to cause a reactive current to flow between the supply voltage and a ground voltage, possibly resulting in a lower power efficiency. Moreover, if a parasitic transistor of the MOS transistor latches up, the IC will be heated so as to possibly cause a failure.
On the other hand, the charge pump circuit is generally used for a load which consumes a relatively small current and supplies a charge accumulated on a capacitor, so that no protection circuit is provided against an over-current to the load. Also, since the charge pump circuit includes a capacitor having a relatively large capacitance, a large rush current is generated upon power-on.
JP-A-10-14218 discloses a method of preventing rush current while limiting a reduction in power conversion efficiency. Specifically, JP-A-10-14218 employs a P-channel type MOS transistor which is turned on when a capacitor is charged. The P-channel MOS transistor is applied with an inverted version of an output voltage at a gate from a charge pump circuit. Therefore, when the charge pump circuit outputs a low voltage, a large rush current can be prevented from flowing because the P-channel type MOS transistor has a large impedance.
Also, the impedance of the P-channel MOS transistor is reduced as the voltage of the charge pump circuit is increased, such that the P-channel type MOS transistor is fully turned on before the output voltage of the charge pump circuit reaches a predetermined value. With this configuration, a power loss can be reduced even if a resistor is inserted for preventing the rush current.
However, the charge pump circuit disclosed in JP-A-10-14218 can have a long rising time because the capacitor is applied with a small charging current when the charge pump circuit outputs a low voltage. Also, when the output voltage of the charge pump circuit is reduced due to an over-current from an output terminal, short-circuiting of a load, or the like, a small current flows from the P-channel type MOS transistor. However, because of an unknown load current at which the output current of the P-channel type MOS transistor begins decreasing, the charge pump is not suitable for use with a load which largely fluctuates in current. Further, the high dependency of the output voltage of the charge pump circuit on a supply voltage still remains unsolved.